Method of evaluating whiteness, method of evaluating comparative whiteness, light source and luminaire

ABSTRACT

In evaluating whiteness of light from a light source or a luminaire, whiteness W is given by the following equation,
 
 W =−5.3 C +100,
         wherein chroma C is determined by the CIE 1997 Interim Color Appearance Model (Simple Version).

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a method of evaluating whiteness oflight from a light source, a method of evaluating comparative whiteness,a light source and a luminaire that produce light with whitenessevaluated as being high by the method of evaluating whiteness.

(2) Related Art

Development of light sources and luminaries has conventionally beendirected toward reproducing the original colors of an object to beilluminated faithfully. Specifically, luminaire that can provide colorappearance of an object closer to that provided under a standardilluminant have achieved a high reputation. This can be objectivelyevaluated using the general color rendering index.

In recent years, however, desirable color appearance has been drawingattention instead of fidelity of color appearance. As a result,techniques for developing light sources and luminaries to render lightcolors as desired for specific applications are receiving attention.Some lamps have already been developed, such as those for making foodsplaced on the shelf look good or making plants at flower stores lookmore beautiful. To evaluate the desirable color appearance, visualclarity index is proposed in Japanese Laid-Open Patent ApplicationNo.9-120797, which indicates how vivid an object color is rendered.

It is pointed out that light colors affect our perception of brightness(See Urabe et al., “Color Temperature of Light Source for InteriorLighting . . . The Effects on Brightness, (1) Impression from outside byMethod of Paired Comparisons”, Annual Conference of The IlluminatingEngineering Institute of Japan, 1995.). In other words, the whiter awhite object is rendered, the brighter the object appears to the humaneyes. Also, as the feeling of whiteness increases, the Figure of anobject looks neat. This would help make the visual environment morecomfortable. Accordingly, it is desirable to use a luminaire with highwhiteness while maintaining the vividity of color appearance.

However, evaluation measurements of such whiteness have conventionallyrelied on subjective judgments. For the same reasons, users have foundit difficult to choose lighting devices.

SUMMARY OF THE INVENTION

The first object of the present invention is to provide a method ofevaluating whiteness objectively.

The second object of the present invention is to provide a light sourceand a luminaire that realize lighting environments with high whitenessand vivid color appearance.

The first object can be achieved by a method of evaluating whiteness oflight emitted from a light source, comprising the steps of: calculatingchroma C, using a method defined by the CIE 1997 Interim ColorAppearance Model (Simple Version); and calculating whiteness W from thechroma C using an equation (1),W=aC+b  (1)

where the coefficient a is a negative real number and the coefficient bis a positive real number. The second object can be achieved by a lightsource, being characterized by: emitting light whose whiteness is nosmaller than 85 and whose visual clarity index is no smaller than 110,the whiteness W being calculated using chroma C of the light and anequation (3),W=−5.3C+100  (3)

wherein the chroma C is calculated using a method defined by the CIE1997 Interim Color Appearance Model (Simple Version).

According to the present invention, the whiteness of light from a sourcecan be evaluated objectively. Also, lighting environments with highwhiteness and vivid color appearance can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, advantages and features of the invention willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings that illustrate specificembodiments of the invention.

In the drawings:

FIG. 1 shows a construction of a whiteness evaluating apparatusaccording to the first embodiment of the present invention;

FIG. 2 is a flowchart describing the steps that the apparatus takes;

FIG. 3 is a graph showing a correlation of chroma with whiteness, wherethe chroma is determined by the CIE 1997 Interim Color Appearance Model(Simple Version), while whiteness is given in a subjective evaluation;

FIG. 4 shows an whiteness evaluating apparatus according to the secondembodiment of the present invention;

FIG. 5 shows a region on the CIE 1931 chromaticity diagram where thewhiteness evaluation apparatus relating to the first embodiment assignswhiteness of 85 or greater;

FIG. 6 shows a region on the CIE 1931 chromaticity diagram where awhiteness evaluation apparatus relating to the third embodiment assignswhiteness of 65 or greater;

FIG. 7 is a graph showing the relation between the ratio Qg/Qv and thevisual clarity index;

FIG. 8 is a graph showing the relation between the ratio Qg/Qv andreciprocal correlated color temperature (T⁻¹), for a fluorescent lampwhose visual clarity index is 110;

FIG. 9 is a graph showing the relation between the ratio Qg/Qv and thereciprocal correlated color temperature (T⁻¹), for a fluorescent lampwhose visual clarity index is 115;

FIG. 10 shows a cross-section of a fluorescent lamp relating to thepresent invention;

FIG. 11 is a table showing phosphor combination examples, #1 to #3;

FIG. 12 shows a spectrum of light emitted from a 30W circulinefluorescent lamp that uses the phosphor combination of #1;

FIG. 13 shows the spectrum of light emitted from a 30W circulinefluorescent lamp that uses the phosphor combination of #2;

FIG. 14 shows the spectrum of light emitted from a 40W slim-linefluorescent lamp that uses the phosphor combination of #3;

FIG. 15 shows a cross-section of a luminaire relating to the presentinvention;

FIG. 16 is a flow chart showing the first procedure of calculatingchroma according to a modification to the invention;

FIG. 17 is a flow chart showing the second procedure of calculatingchroma according to a modification to the invention: and

FIG. 18 is a flow chart showing the first procedure of calculatingchroma according to a modification to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a description of the preferred embodiments of thepresent invention, with reference to the drawings.

1. First Embodiment

The following is a description of a method of evaluating whitenessrelating to a first embodiment of the present invention.

FIG. 1 shows an appearance of whiteness evaluation apparatus 1. As shownin the figure, the whiteness evaluation apparatus 1 is comprised of thespectrophotometer 2 and the personal computer (hereafter referred to asPC) 3 connected to the spectrophotometer 2. The spectrophotometer 2measures spectrum of light from a light source. Receiving spectral datafrom the spectrophotometer 2, the PC 3 calculates chroma and whitenessfrom the data, and then displays the whiteness on the computer screen.

FIG. 2 is a flow chart illustrating the procedure the PC 3 follows toprocess spectral data passed by the spectrophotometer 2, until itcompletes the calculation of whiteness. First, the PC 3 receivesspectral data from the spectrophotometer 2 (Step S1). Secondly, the PC 3calculates chroma C in accordance with the definition of The CIE 1997Interim Color Appearance Model (Simple Version) (hereafter referred toas CIE Model. CIE stands for Commission Internationale de Léclairage)(Step S2). Thirdly, it substitutes the chroma for C in the followingequation (A) to produce whiteness W (Step S3).W=−5.3C+100  (A)

Finally, the PC 3 displays whiteness W obtained from Step S3 on thescreen (Step S4), and completes the processing.

It is understood that a light source that gives a feeling of higherwhiteness has relatively lower chroma. In an experiment using the samelight sources and color chips, it was found that a correlation existsbetween subjective evaluation of whiteness and chroma defined by theabove CIE Model. The subjective evaluation is determined by subjects inan experiment, where they evaluate how white color chips appear underlight sources on the scale from 0 to 100. When a subject feels that anobject color is totally white, he or she scores 100, while an objectcolor doesn't look white at all, it is given 0. In this way, the abovesubjective evaluation values were obtained.

FIG. 3 is a scatter diagram to which the data obtained in the aboveexperiment was plotted. The vertical axis represents subjectiveevaluation values and the horizontal axis represents chroma values. Asshown in the diagram, the subjective evaluation values and the chromahave the relationship of a linear function that has a slope of anegative number. Here, the correlation coefficient between the chromavalues and subjective evaluation values is 0.93. This demonstrates thatsubjective evaluation values are roughly proportional to chroma values.Therefore, by defining whiteness W using a linear expression of chromaC, as follows,W=aC+b

it is possible to evaluate whiteness objectively, without performing asubjective evaluation test.

The CIE Model was proposed by the CIE, so as to provide several indiceswith the highest accuracy among several other color appearance models.Using the model, various indices can be derived such as chroma, hue andbrightness. Although many other models are available for quantifyingchroma, the CIE Model is preferable for evaluating whiteness.

In the above equation (A), the coefficients a and b are determined sothat a standard illuminant A with an illuminance of 500 lux has awhiteness of 50 and a light source with a chroma of 0 has a whiteness of100. By such determining the coefficients a and b, whiteness of almostall general-purpose light sources can be contained in the range of 0 to100. This is quite useful for evaluating light sources. Since chroma Calways takes 0 or a positive value, with the equation (A) it is possibleto evaluate characteristics of light sources on a scale from 0 to 100.Also, in this embodiment, the spectrophotometer 2 receives lightdirectly from a light source and measures its spectrum. This isanalogous to the case where the human eyes view an ideal white objectwhose spectral reflectance is 1.0 for visible light regions of 380 nm to780 nm in the CIE Model.

2. Second Embodiment

The following is a description of a method of evaluating whitenessrelating to the second embodiment of the present invention. In thisembodiment, color chip N9.5 (manufactured by the Japan Color ResearchInstitute) is used as an object to evaluate whiteness W of light from alight source. The color chip N9.5 is an available object whose spectralreflectance is the closest to a spectral reflectance 1.0 of an idealwhite object. The Munsell value and Munsell chroma of this color chip is9.5 and 0, respectively. Since the Munsell chroma is 0, Munsell hue isdefined for the color chip.

FIG. 4 shows a construction of an apparatus for evaluating whiteness Wof light 5 given off from a light source 4, using a color chip 6 as anobject. Light 5 from a light source 4 is reflected on the surface of thecolor chip 6 before it is received by the spectrophotometer 2. Thespectrophotometer 2 generates spectral data and sends it to a PC (notshown in the figure). The PC uses this spectral data to calculate chromaC. The PC then substitutes chroma C into the following equation,

 W=−4.4C+100

to produce whiteness W for the light. The coefficients of the equationare determined so that a standard illuminant with an illuminance of 500lux has a whiteness of 50 using of the color chip as an object, and thatthe light with a chroma of 0 has a whiteness of 100.

3. Third Embodiment

The following is a description of a method of evaluating whitenessrelating to the third embodiment of the present invention. In thisembodiment, a blank surface of a newspaper or a magazine, which is oftenviewed in daily life, is used as a visual object to evaluate whitenessof a light source. Such a blank surface has a Munsell value of about8.0, a Munsell chroma of about 1.0 and a Munsell hue of about 5GY, whichare equivalent to those of color chip 5GY 8.0/1.0 (manufactured by theJapan Color Research Institute). Accordingly, in this embodiment thiscolor chip is used as an object to evaluate whiteness of light from alight source. The procedure used in this embodiment is roughly the sameas that in the second embodiment, except that an equation used by a PCto calculate whiteness W from chroma C isW=−3.3C+100

The two coefficients of this equation are determined so that a standardilluminant A with an illuminance of 500 lux has a whiteness of 50 usingthe color chip as an object, and that the standard illuminant A has witha chroma of 0 has a whiteness of 100.

The experiment showed that more than 50 percent of the subjects couldn'tdistinguish one light source from the other when a comparative whitenessbetween the two sources was no greater than 15 percent. Put another way,for more than 50 percent of the subjects, there is no difference betweena light source with a whiteness of 85 and a light source with awhiteness of 100. The ellipse in FIG. 5 represents a region on the CIE1931 chromaticity diagram, where whiteness of 85 or greater is shownusing the apparatus of the first embodiment. This is the inside area ofan ellipse which centers at chromaticity coordinates (x,y)=(0.3400,0.3390), has the major axis and minor axis of 0.0150 long and 0.0060long respectively, with the major axis inclining against the x-axis atan angle of 45°. Also, due to the characteristics of the objects usedhere, the minimum chroma value exceeds 0, and the maximum whitenessvalue is 75. Again, more than 50 percent of the subjects couldn'tdistinguish a source with a whiteness of 75 from the other light sourcewhen a comparative whiteness between the two sources was no greater than15 percent. This means that a light source with a whiteness of 65 isevaluated to be the same as a light source with a whiteness of 75. Asshown in FIG. 6, a region with a whiteness of 65 or greater as in theabove case forms an ellipse on the CIE 1931 chromaticity diagram. Thearea is the inside of an ellipse which centers at chromaticitycoordinates (x,y)=(0.3100, 0.2600), has the major axis and minor axis of0.1050 long and 0.0400 long respectively, and with the major axisinclining against the x-axis at an angle of 52 degrees.

Light colors of light sources generally used for lighting are known tobe within a range of y≧−2.63x²+2.63x−0.263, y≧−3.09x+1.22, on the CIE1931 chromaticity diagram. A diagonally shaded area in FIG. 6 representa region which is evaluated to have a whiteness of 65 or greater by thewhiteness evaluation apparatus of the third embodiment and which can beapplied to general-purpose light sources. Due to the human visualcharacteristics, even with the same intensity of whiteness, while alight in bluish color can be used for commercial applications, a lightin other colors including red and reddish colors can't serve for thepurpose.

Regarding the visual clarity index proposed in Japanese Laid-Open PatentApplication No.9-120797, it is reported that brightness of 1.1 times ofa standard illuminant is perceived under a light source with a visualclarity index of 110, and when the index is 115, the brightnessincreases to 1.15 (See Hashimoto et al., “New Method of Specifying ColorRendering Properties of Light Sources based on the Feeling of Contrast”,Journal of Illuminating Engineering Institute of Japan, pp.639-671,Vol.79, No.11, 1995).

In sum, desirable light sources should have whiteness and visual clarityindex of values as follows. When evaluating a whiteness of a lightsource, a color of light from the source should preferably be locatedwithin an ellipse, which centers at the chromaticity coordinates(x,y)=(0.3400, 0.3390), has major axis and minor axis of 0.0150 long and0.0060 long respectively, with the major axis inclining against thex-axis at an angle of 45 degrees. At the same time, it is preferable ifits visual clarity index is at 110 or greater, and more preferably at115 or greater. When using newspaper as an object, it is desirable if alight color of a light source is specified within an ellipse thatcenters at the chromaticity coordinates (x,y)=(0.3100, 0.2600), wherethe length of major axis and minor axis is 0.1050 and 0.0400respectively, and the major axis is inclining against the x-axis at anangle of 52 degrees, and the light color is within a range ofy≧−2.63x²+2.63X−0.263, y≧−3.09x+1.22. Its visual clarity index shouldpreferably be at 110 or greater, and more preferably, at 115 or greater.

Under a three band fluorescent lamp, which is commonly used as ageneral-purpose light source, it was found that objects in severalcolors get more vivid and brighter, as radiant energy in a wavelengthrange from 505 nm to 530 nm increases against radiant energy in awavelength from 380 nm to 780 nm. FIG. 7 is a graph showing therelationship between the visual clarity index and a ratio Qg/Qv under afluorescent lamp with a correlated color temperature of 5200 [K]. Theratio Qg/Qv refers to a ratio between radiant energy in a wavelengthfrom 380 nm to 780 nm (visible band) (Qv) and radiant energy in awavelength from 505 nm to 530 nm(Qg). As can be seen from FIG. 7, Qg/Qvvaries in direct proportion to the visual clarity index. It was alsofound that the relationship between Qg/Qv and the visual clarity indexvaries according to the correlated color temperature.

FIG. 8 shows a graph showing the relationship between the ratio Qg/Qvand reciprocal correlated color temperature T⁻¹ [K⁻] under a fluorescentlamp with a visual clarity index of 110. As shown in FIG. 8, thereciprocal correlated color temperature T⁻¹[K⁻¹] and Qg/Qv have arelationship of a linear function that has a slope of a negative number.As shown in FIG. 8, FIG. 9 shows a graph that describes the relationshipbetween Qg/Qv and reciprocal correlated color temperature T⁻¹[K⁻¹] undera fluorescent lamp with a visual clarity index of 115. Again, they havea relationship of a linear function that has a slope of a negativenumber.

In conclusion, when Qg/Qv≧−0.11×10⁴ T⁻¹+0.30, visual clarity index of afluorescent lamp is 110 or greater, and when Qg/Qv≧−0.1×10⁴ T⁻¹+0.33,the index is 115 or greater. Therefore, by using the ratio Qg/Qv and thereciprocal correlated color temperature instead of visual color index,it will become easier to evaluate color appearance of an object.

It is also useful to be able to evaluate a whiteness of most of thegeneral-purpose light sources in a score range of 0 to 100, when awhiteness under a standard illuminant A is 50 and a whiteness with achroma of 0 is 100.

Forth Embodiment

The following is a description of how to achieve a light source of thecharacteristics, raising a fluorescent lamp as an example. Having highefficiency and color rendering properties, three band fluorescent lampsare used widely for lighting in houses, stores and offices. The threeband fluorescent lamps contain rare earth phosphors that produce spectrain three wavelength ranges of 440 nm to 470 nm, 540 nm to 570 nm and 600nm to 620 nm, where the strongest chromatic response is produced.Phosphors are blended together so that high energy is produced in awavelength with high spectral luminous efficiency. To make a fluorescentlamp of the characteristics as mentioned above, another phosphor havingspectrum at 505 nm to 530 nm should be added to the three phosphors.

Phosphor Composition

For a phosphor whose emission peak is located within a range from 440 nmto 470 nm, at least one of the phosphors containing bivalent europium asan emission center should be used. Phosphors in this group are selectedfrom the group consisting of

-   -   BaMgAl₁₀O₁₇:Eu²⁺,    -   BaMgAl₁₀O₁₇:Eu²⁺,Mn²⁺ and    -   (Ba,Ca,Sr,Mg)₁₀(PO₄)₆Cl₂:Eu²⁺.        In a phosphor containing BaMgAl₁₀O₁₇ as a host crystal such as    -   BaMgAl₁₀O₁₇:Eu²⁺ and    -   BaMgAl₁₀O₁₇:Eu²⁺,Mn²⁺        a portion of barium elements in the compounds could be replaced        with another alkali earth metal element, such as calcium or        strontium. By doing so, it is possible to shift the wavelength        of peak emission of Eu²⁺ or change the half-width of it.

For a phosphor whose emission peak is located within a range from 505 nmto 530 nm, at least one of the phosphors containing bivalent manganeseas an emission center should be used. Phosphors in this group areselected from the group consisting of

-   -   BaMgAl₁₀O₁₇:Eu²⁺,Mn²⁺,    -   CeMgAl₁₁O₁₉:Mn²⁺,    -   Ce(Mg, Zn)Al₁₁O₁₉:Mn²⁺,    -   Zn₂SiO₄:Mn²⁺, and    -   CeMgAl₁₁O₁₉:Tb³⁺,Mn⁺.

For a phosphor whose emission peak is located within a range from 540 nmto 570 nm, at least one of the phosphors containing trivalent terbium asan emission center should be used. Phosphors in this group are selectedfrom the group consisting of

-   -   LaPO₄:Ce³⁺,Tb³⁺ and    -   CeMgAl₁₁O₁₉: Tb³⁺.

For a phosphor whose emission peak is located within a range from 600 nmto 620 nm, at least one of the phosphors containing trivalent europiumas an emission center should be used. Phosphors in this group areselected from the group consisting of

-   -   Y₂O₃:Eu³⁺ and    -   Gd₂O₃:Eu³⁺.

By combining four or more phosphors whose emission peaks are locatedwithin either of the four bands, a phosphor layer is formed and afluorescent lamp of the present invention is realized.

It is also appreciated to employ a phosphor that has emission peaks intwo of the four wavelength ranges. Since the phosphor of this kind cangenerate radiation in two wavelength ranges at a time, fewer phosphorsare needed to form a phosphor layer.

Phosphors that emit radiation in two wavelength ranges include rareearth ions such as bivalent europium and trivalent cerium, and phosphorshaving bivalent manganese. When a fluorescent lamp is turned on,ultraviolet light is generated, whose wavelength is 253.7 nm, and therare earth ions absorb and convert the light into visible light or lightnear UV spectrum. Some of the light energy is transmitted to thebivalent manganese, which in turn releases the energy in the form oflight having a peak emission at 505 nm to 530 nm. Since bivalentmanganese doesn't absorb ultraviolet radiation much, spectra of thelight can be adjusted by changing the ratio of the rare earth ions tothe bivalent manganese.

Phosphors having peak emissions both at 440 nm to 470 nm and 505 nm to530 nm include phosphors that have bivalent europium and bivalentmanganese. For example, the spectrum to be produced from a phosphorcontaining BaMgAl₁₀O₁₇:Eu², Mn²⁺ can be adjusted by changing the amountof the bivalent europium or the bivalent manganese. As a percentage ofbivalent manganese increases against bivalent europium, the bivalenteuropium produces lesser light having a peak at 440 nm to 470 nm. On theother hand, the bivalent manganese produces more light whose peakemission is at 505 nm to 530 nm, until the radiation from the bivalenteuropium becomes invisible and only the bivalent manganese produce lighthaving a peak emission at 505 nm to 530 nm.

Phosphors having a peak emission at both 505 nm to 530 nm and 540 nm to570 nm are phosphors that contain trivalent terbium and bivalentmanganese, including CeMgAl₁₁O₁₉:Tb³⁺, Mn²⁺. In the case ofCeMgA₁₁O₁₉:Tb³⁺,Mn²⁺, trivalent cerium absorbs ultraviolet radiation,and then it transmits energy to trivalent terbium and bivalentmanganese, which in turn produce light having a peak emission at 540 nmto 570 nm and light having a peak emission at 505 nm to 530 nm,respectively. At the same time, trivalent cerium produces light havingan emission peak at 300 nm to 400 nm, but it is almost impossible toperceive the light because of its lower luminous efficiency. Therefore,visible light is light mainly from trivalent terbium and bivalentmanganese. Since the trivalent cerium transmits energy to both thetrivalent terbium and the bivalent manganese, spectrum of the light tobe produced is adjustable by changing the amount of trivalent terbiumand bivalent manganese, as long as the same percentage of trivalentcerium is contained.

In this specification, a group of three or more kinds of phosphorshaving emission peak in the wavelength ranges mentioned above arereferred to as constituting a main component, when they account for 70to 100 weight percent of all the phosphors contained in a phosphorlayer. In other words, it is possible to add another phosphor as long asit doesn't exceed 30 percent of the entire phosphor layer, by weight. Toimprove color rendition, for example, phosphors are added whose emissionpeak is at 620 nm to 670 nm or in any other regions in which the colorrendering effect improves. It is appreciated to add such phosphors inorder to improve color rendition.

Building a Fluorescent Lamp

The following is an explanation of how to build a fluorescent lamp usingthe above phosphors. We take a fluorescent lamp 7 in FIG. 10. FIG. 10shows a longitudinal section of the fluorescent lamp 7. The fluorescentlamp 7 contains a phosphor layer 9 on the interior surface of a glasstube 8, which is sealed at both of the ends with stems 10. The stems 10have lead wires 14 penetrating through them hermetically. The lead wires14 are connected with filaments 13, and bonded to electrodes 12 whichare supported by caps 11.

After blended in a specified proportion, the phosphors are adjusted andmixed with organic solvent, such as water and butyl acetate, and produceslurry. Aqueous polymers or polymers soluble in an organic solvent canbe added to the mixture, so as to make the forming of films easier. Abinding agent can also be added to strengthen bonds between phosphorsand between a phosphor and a glass tube 8.

Then, the slurry is applied to the inside surface of the glass tube 8and dried to form a phosphor layer 9. When it is confirmed that thephosphor layer 9 is formed, inert gases (such as argon gas) and mercuryare flown into the glass tube 8, whose ends are then sealed with thestems 10. The caps 11 are bonded to the end of the glass tube 8, and theelectrodes 12 and lead wires 14 are connected to the caps 11. In thisway, the fluorescent lamp 7 relating to the present invention isachieved.

The method of building a slim-line fluorescent lamp is applicable tomaking circuline lamps and lamps in other shapes, on condition that thephosphors are added in a specified proportion.

FIG. 11 shows three types of fractional weight percent of the phosphorcomponents #1 to #3. Phosphors in #1 consist of 19 percent ofBaMgAl₁₀O₁₇:Eu²⁺, 29 percent of Ce (Mg, Zn) Al₁₁O₁₉:Mn²⁺, 35 percent ofLaPO₄:Ce³⁺,Tb³⁺, and 27 percent of Y₂O₃:Eu³⁺, by weight.

Phosphors in #2 consist of 42 percent of BaMgAl₁₀O₁₇, 15 percent ofLaPO₄:Ce³⁺,Tb³⁺, 43 percent of Y₂O₃:Eu³⁺, by weight.

Phosphors in #3 consist of 39 percent of(Ba,Ca,Sr,Mg)₁₀(PO₄)₆Cl₂:Eu²⁺,24 percent of Ce(Mg, Zn)Al₁₁O₁₉:Mn²⁺, 4 percent of LaPO₄:Ce³⁺,Tb³⁺, and33 percent of Y₂O₃:Eu³⁺, by weight.

FIG. 12 shows a spectrum of radiation emitted from a 30W circulinefluorescent lamp (FCL30) that contains phosphors as shown in #1. By thewhiteness evaluating apparatus used in the first embodiment, the lamp isevaluated to have a whiteness of 90.9, visual clarity index of 111 andchromaticity coordinates (x,y)=(0.3393, 0.3420). This means that thefluorescent lamp can provide an ideal light color. That its correlatedcolor temperature is 5204[K] and Qg/Qv is 0.09 agrees with the visualclarity index mentioned above.

FIG. 13 shows a spectrum of light emitted from a 30W circulinefluorescent lamp (FCL30) that contains phosphors as shown in #2. By thewhiteness evaluating apparatus used in the first embodiment, the lamp isevaluated to have a whiteness of 93.6, visual clarity index of 115 andchromaticity coordinates (x,y)=(0.3375, 0.3339) on the CIE 1931chromaticity diagram. This means that the fluorescent lamp can providean ideal light color. That the correlated color temperature is 5256[K]and Qg/Qv is 0.15 agrees with the visual clarity index mentioned above.

FIG. 14 shows a spectrum of radiation from a 40W slim-line fluorescentlamp (FL40S) that contains phosphors as shown in #3. By the whitenessevaluating apparatus used in the third embodiment, the lamp is evaluatedto have a whiteness of 68.6, visual clarity index of 110 andchromaticity coordinates (x,y)=(0.3057, 0.3084) on the CIE 1931chromaticity diagram. This means that the fluorescent lamp can providean ideal light color. That the correlated color temperature is 7170 [K]and Qg/Qv is 0.17 agrees with the visual clarity index mentioned above.It is also confirmed from a subjective evaluation that under afluorescent lamp containing phosphors as shown in #3, objects in variouscolors including newspapers and white objects look whiter and morevivid.

FIG. 15 shows a longitudinal section of a luminaire 15 that contains theabove fluorescent lamp. The luminaire 15 is comprised of a fluorescentlamp 16, first carriers 17 supporting the fluorescent lamp 16, areflector 19 that reflects light from the fluorescent lamp 16, and atranslucent cover 18 that allows light from the lamp 16 and reflectedlight from the reflector 19 to go through. The luminaire 15 is fixed onthe ceiling or on the wall with second carriers 20. The reflector 19 ismade from a material that doesn't absorb a particular radiation invisible bands, and therefore has a virtually uniform spectralreflectance in the visible bands. The translucent cover 18 is made froma material that doesn't absorb a particular radiation in visible bands,and therefore has a virtually uniform spectral transmittance in thevisible bands. Using these components, a luminaire of the desiredcharacteristics is achieved.

Even a luminaire lacking the translucent cover 18 can produce desirablelight, with the reflector 19 made from the same materials as mentionedabove. A luminaire having other forms of light sources than fluorescentlamps in it can also produce desirable light, so long as the lightsource have the characteristic requirements.

We have seen embodiments of the present invention by raising fluorescentlamps as a light source of excellent characteristics. But even a lightsource that fails to meet the requirement can produce desirable light,if it has several components. That is, if the light source has areflector and a translucent cover, it is possible to convert light froma light source located inside the luminaire into light of thecharacteristics mentioned above and produce desired light.

Specifically speaking, the translucent cover should be made either fromglass or plastic. A glass translucent cover is made by applying a dosageof metal ions that absorb light in particular wavelength ranges, such asCr³⁺, Mn³⁺, Fe³⁺, Co²⁺, Ni²⁺ and Cu²⁺, to a glass and making a glasstranslucent cover in an intended form. The dosage of the metal ionsshould preferably be 15 mol weight percent or under of the entire glass.A plastic translucent cover is made by blending and kneading someinorganic and organic pigments into plastic materials before making aplastic translucent cover in an intended form. Those inorganic pigmentsinclude cobalt violet, cobalt blue, ultramarine, cobalt green, cobaltchrome green, titan yellow, red iron oxide and minium. As for theorganic pigments, they are selected from a group consisting of dioxanecompound, phthalocyanine compound, azo compound, perylene compound andpyrrolopyrrol compound. The pigment dosage should preferably be 5 weightpercent or under of the entire plastic materials.

Spectral reflectance of a translucent cover can be adjusted by forminglayers of plastic films that contain the light absorbing materials onthe surface of it. Paints that contain the light absorbing materialscoated on the surface of a glass or plastic translucent cover can alsochange the spectral reflectance. This also makes it possible to adjustspectral transmittance of a translucent cover easily. As for thespectral reflectance for a reflector, it can be adjusted by blending thelight absorbing materials into a basic material that constitutes thereflector, or by forming layers containing the light absorbing materialson the surface of the basic material. The translucent cover and thereflector can be employed either in combination or separately, toprovide the effect of the present invention.

We have explained the present invention in accordance with some of theembodiments, but they aren't the only forms of the application. It willbe appreciated that the present invention is realized in othermodifications, some of which are explained below.

Modifications

(1) The following is an explanation of how to obtain chroma C for theembodiments, 1,2,3.

FIG. 16 shows a flow chart describing the processing contents to obtainchroma C. First, a calorimeter measures tristimulus values, X,Y,Z of theXYZ trichromatic system for light from a light source (step S10). Then,the calorimeter measures tristimulus values, X′,Y′,Z′ for reflectedlight from a particular visual object (step S11). Finally, chroma C isobtained by calculating these values in accordance with the CIE Model(step S12).

FIG. 17 shows a flow chart describing another processing contents toobtain chroma C. First, a luminance meter measures spectral distributionof light from a light source (step S20). Based on the spectraldistribution, tristimulus values, X,Y,Z for radiation from a lightsource are determined(step S21). Then, the luminance meter measuresspectral distribution of reflected light from a particular visual object(step S22), and determines tristimulus values, X′,Y′,Z′ for the lightbased on the spectral distribution (step S23). Finally, these values aresubstituted into the CIE Model to obtain chroma C (step S24).

There's another way to obtain chroma C. FIG. 18 shows a flow chartdescribing processing contents to obtain chroma C. As is described inFIG. 17, in FIG. 18 a luminance meter measures spectral distribution oflight from a light source (step S30), and determines tristimulus values,X,Y,Z for the light based on the spectral distribution. Then, aspectrophotometer measures spectral reflectance of the object (stepS33). Based on the spectral distribution and the spectral reflectance,spectral distribution of light reflected from the object is calculated(step S33). When tristimulus values X′,Y′,Z′ are determined based on thespectral distribution (step S34), these values are calculated inaccordance with the CIE Model to obtain chroma C (step S35).

As a colorimeter, recommended are BM-5A (TOPCON Corporation) andluminance meters, CS-1000 (Minolta Co., Ltd.) and SR-3 (TOPCONCorporation). As a spectrophotometer, CM-3530 (Minolta Co., Ltd.) isrecommended.

(2) Whiteness, given by using the methods mentioned above, is whitenessmeasured on the interval scale. In such a case, comparative whitenessbetween 70 and 80 is equivalent to comparative whiteness between 80 and90.

If a label is displayed on a lamp to show whiteness on the ratio scale,together with whiteness on the interval scale, it helps consumerscompare whiteness of one lamp with whiteness of another. A ratio scale,comparative whiteness Wc, would best suites for the purpose, which isthe following.

To obtain comparative whiteness Wc between two light sources, the firststep is to obtain chroma C1 for light from a first light source andchroma C2 for light from a second light source, according to the CIEModel. Then, those chroma values are substituted into the followingequation (B).Wc=(C 1 −C 2)/C 1  (B)

In this way, an objective ratio scale, independent of a subjectiveevaluation method, can be obtained.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art.

Therefore, unless such changes and modifications depart from the scopeof the present invention, they should be construed as being includedtherein.

1. A method of evaluating whiteness of light emitted from a fluorescentlamp, comprising the steps of: calculating chroma C of light emittedfrom a fluorescent lamp, using a method defined by the CIE 1997 InterimColor Appearance Model (Simple Version); and calculating whiteness Wfrom the chroma C using an equation (1),W=aC+b  (1) where the coefficient a is a negative real number and thecoefficient b is a positive real number.
 2. The method of claim 1,wherein the coefficient a is −5.3 and the coefficient b is
 100. 3. Themethod of claim 1, wherein the chroma C is a chroma of light obtainedwhen the light from the fluorescent lamp is reflected off from a surfaceof an object whose Munsell value and Munsell chroma is 9.5 and 0,respectively, and the coefficient a is −4.4 and the coefficient b is100.
 4. The method of claim 1, wherein the chroma is a chroma of lightobtained when the light emitted from the fluorescent lamp is reflectedoff a blank surface of a newspaper, and the coefficient a is −3.3 andthe coefficient b is
 100. 5. A method of evaluating whiteness of lightemitted from a fluorescent lamp, comprising the steps of: calculatingchroma C of light emitted from a fluorescent lamp, using a methoddefined by the CIE 1997 Interim Color Appearance Model (Simple Version);and calculating whiteness W from the chroma C using an equation (1),W=aC+b  (1) where the coefficient a is a negative real number, thecoefficient b is a positive real number, and the whiteness W is 100 whenthe chroma C is
 0. 6. A method of evaluating whiteness of light emittedfrom a fluorescent lamp, comprising the steps of: calculating chroma Cof light emitted from a fluorescent lamp, using a method defined by theCIE 1997 Interim Color Appearance Model (Simple Version); andcalculating whiteness W from the chroma C using an equation (1),W=aC+b  (1) where the coefficient a is a negative real number, thecoefficient b is a positive real number, the whiteness W is 100 when thechroma C is 0, and the whiteness W is 50 under a standard illuminant A.7. A method of evaluating comparative whiteness of light emitted fromtwo light sources, comprising the steps of: calculating chroma C1 oflight from a first light source and chroma C2 of light from a secondlight source using a method defined by the CIE 1997 Interim colorAppearance Model (simple version); and calculating comparative whitenessWc from the chroma C1 and the chroma C2, using an equation (2),Wc=(C 1-C 2)/C 1  (2).